Automatic signal interference control



March 28, 1939. H WHEELER 2,152,515

AUTOMATIQ SIGNAL- INTERFERENCE CONTROL Filed June 18, 1937 s Sheets-Sheet 1 Fl'G.l.

n [l0 f I 0 C S CARRIER AMPLIFIERC om AND SELECTOR $55935 g; WITH LEvEL Ave-*- FREQUENCY DETECTOR RECTIFIER o 9 9 l I FIG.2.

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o o c o CARRiER AMPLIFIERG A AND SELECTOR gz'ig gg" Q WITH LEVEL Ave o O a c o 9 o- L. l l2 Lu-o V C o ANPLlTUDE DETECTOR RECTIFIER Q C 0 T March 28, 1939. H. A. WHEELER 2,152,515

AUTOMATIC SIGNAL- INTERFERENCE CONTROL Filed'June 18, 193'? 3 Sheets-Sheet 5 48 4 154 sa 49 so INVENTOR HA LD A. WHEE R BY ATTORNEY u Briefly stated, this object is achieved by pro- Patented Mar. 28, 1939 UNITED STATES AU'roMArIc SIGNAL m'marnnnucn common Harold A. Wheeler, Great Neck, N. r., assignor to Hazeltine Corporation, a corporation of Delaware Application June is, 1937, Serial No. 148,894

18 Claims.

This invention relates to modulated-carrier signal receivers and, more particularly, to methods of, and means for, controlling the selectivity and fidelity of reproduction of such receivers to reduce the effects of undesired signal disturbances.

In the present system of broadcasting, different signals are transmitted on carrier frequencies usually separated by multiples of 10 kilocycles, while the modulation sidebands of each signal cover ranges of frequency extending as much as 8 kilocycles above and below their respective carrier frequencies. In order to obtain the highest possible fidelity of reproduction of the desired signal consistent with the conditions of reception, it is necessary that various characteristics of the receiver be so controlled as to reduce the effects of undesired signal disturbances. These disturbances may be of various types which, for convenience, may be divided into two classes: (1) those which are substan- I tially continuous, including steady noise having its power distributed over the audio-frequency range and beat-note interference resulting from modulation of the desired signal carrier by carriers of undesired signals adjacent the desired signal carrier; and (2) disturbances of an intermittent or discontinuous type, including electrical transients or sharp impulses and distortion of the desired signal due to detuning during the tuning operation.

Heretofore, various systems have been devised for automatically controlling the selectivity and fidelity of reproduction of radio receivers in accordance with received signal conditions. In all of such systems in which the control has been efiected in accordance with undesired signal disturbances, it has been dependent, at least in part, upon the amplitude of such disturbances and these disturbances could be differentiated from the desired signal only when their amplitude was of the samelorder of magnitude as the desired signal amplitude. It is desirable, however, to

provide a control of the receiver characteristics such as will reduce the effects of undesired signal disturbances even when they are materially weaker than the desired signal.

It is an object of the present invention, therefore, to provide an improved method of, and means for, controlling the characteristics of a modulated-carrier signal receiver to reduce the eifects olf undesired signal disturbances. regardless of their amplitude relative to that of the desired signal.

viding a modulated-carrier receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal, but subject to modulation of said first type as well as modulation of a second characteristic type by undesired signal disturbances, together with means for selectively detecting and utilizing only the modulation of the first type to reproduce the desired signal. Means are further provided for selectively detecting and utilizing only the modulation of the second type to so control the characteristics of the receiver as to reduce the effects of the undesired signal disturbances. Thus, the present invention is based upon the principle that signal disturbances, in general, cause several types of modulation of the desired signal carrier, for example, both amplitude and frequency modulation. While it is theoretically possible for a disturbance to produce only one kind of modulation at a given instant, these different types of modulation are ordinarily produced at the same time and their probable coincidence makes it possible to obtain adequate control by operating controlcircuits in response to modulation of the type which is not being utilized to reproduce the desired signal. In this specification and in the appended claims, the term type of modulation is utilized to designate the ,manner in which a carrier wave is varied by a desired signal or by undesired signal disturbances. For example, variations of one characteristic of a carrier wave, such as its amplitude, constitute modulation of the amplitude type, while variations of a different characteristic of a carrier wave, such as its frequency, constitue modulation of the frequency type. That is, the term does not refer to the type of signal effecting the modulation but to the particular manner in which any desired or undesired signal modulates a carrier wave, that is, to the characteristics whichv are modulated.

In accordance with one feature of the invention, the control means are designed to be responsive only to modulation of the desired signal of the second type by substantially continuous disturbances. The receiver includes a band-pass selector for selecting the desired signal and the control means serves to adjust the width of the band of frequencies passed by the selector in accordance with the amplitude of the undesired signal disturbances, thereby to reduce the eflects thereof. The adjustment of band width is preferably obtained in the carrier-frequency channel of the receiver, but the equivalent effect may also be obtained by proper adjustment of the width of the audio-frequency band passed by the system. The continuous disturbances may, for example, be steady noise, whlch is understood to include rapidly recurring clicks, or they may comprise beat-note signals such as are developed by modulation of the desired signal carrier by the carrier of a signal adjacent the desired signal and, in this case, the control means may inelude means selectively responsive only to these beat-note signals for effecting the band width adjustment.

In accordance with another feature of the invention, the control means is designed to be responsive to undesired Signal disturbances of an intermittent or discontinuous type and operates to suppress the reproduction of such undesired signals, as well as the desired signals, for the duration of the disturbance. These discontinuous disturbances may, for example, be electrical transients of very short duration, so that the suppression of the reproduction does not appreciably disturb the program, or they may constitute undesired distortions of the desired signal such as are developed in the receiver when detuned during the tuning operation. v

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings, Fig. 1 is a schematic diagram of a radio receiver embodying one form of the present invention; Fig. 2 is a similar diagram illustrating another form of the invention; Fig. '3 is a schematic diagram of a complete superheterodyne receiver based on the fundamental arrangement illustrated in Fig. 1 and including control circuits embodying various features of the present invention; and Fig. 4 is a circuit diagram showing representative circuits which may be used in the portions of the receiver of Fig. 3 embodying the present invention. a

Referring now'more particularly to Fig. 1, there is shown schematically a receiver embodying a carrier-frequency amplifier and selector l having its input circuit connected with an antenna II and ground I2. A main signal-detecting, amplifying, and reproducing channel is connected to the output circuit of the amplifier and selector Iliand includes, in the order named, an amplitude-modulation detector II, a modulation amplifier i4, and loud-speaker ii, in accordance with conventional practice.

The vportion of the receiver described being conventional, a detailed description thereof is unnecessary. Briefly, however, a carrier amplitude modulated by a desired signal is intercepted by the .antenna, selected and amplified in the amplifier I0, and delivered to the amplitude modulation detector i3, wherein the audio-frequency signal is derived. The latter signal is further amplified in the amplifier H and supplied in the usual manner to the loud-speaker l5 fot reproduction.

In accordance with the present invention, a control channel is connected to the output circuit of the amplifier and selector i0 and includes, in the order named, a frequency detector l5 and rectifier l'l. Certain of the circuits of the main channel of the receiver are arranged to be so controlled as to reduce the eflects of undesired signal disturbances, as, for example,

by adjusting the width of the band of desired signal frequencies passed by the system or by entirely suppressing the reproduction of both the desired and undesired signals for the duration of the latter. While such arrangements are explained in detail in connection with the description which follows with respect to Figs. 3 and 4, in the present instance, for the purpose of brevity, such circuits, per se, are not shown, but suitable circuits of this type will be considered as included in the amplifier, and selector l0 and the amplifier l4, adapted for adjustment by means of control-bias voltages. The output of the rectifier I1 is connected by suitable leads i8 and I9 to the amplifier and selector I0 and to the amplifier l4, respectively, to supply these control-bias voltages.

In the operation of the system shown in Fig. l, the desired amplitude-modulated signal is detected by the amplitude-modulation detector l3 and amplified and reproduced in the manner described above. Undesired signal disturbances generally effect both amplitude and frequency modulation of the desired signal, however, and the modulation of the latter type only is selectively detected by the frequency-modulation de- 1 tector l8 and rectified by the rectifier H to develop control-bias voltages variable in accordtics of the receiver as to reduce the undesired signal disturbances.

In Fig. 2 there is illustrated a modified form of the invention designed for the reception of carrier signals frequency-modulated by the desired signal. The receiver of Fig. 2 is similar to that of Fig. 1, except that here the main channel includes a frequency-modulation detector l3a, instead of the amplitude-modulation detector II of Fig. l, and the control channel includes an amplitude-modulation detector l6a, i stead of the frequency-modulation detector i Other corresponding parts of the two receivers are incheated by the same reference numerals. The only difference in the operation of the receiver of Fig.2 with respect to that of Fig. 1 is that in the arrangement of Fig. 2the desired signal is solely frequency-detected in the main channel and the undesired signal disturbances are solely and selectively amplitude-detected in the control channel to develop the control-bias voltages.

Referringnow to Fig. 3, there is shown a complete superheterodyne radio receiver embodying the present invention for effecting various different types of control in accordance with various kinds of undesired signal disturbances. In general, the receiver of Fig. 3 includes a tunable radio-frequency amplifier and frequency changer 20, having its input circuit connected to an antenna 2i and ground 22. Connected to the output circuit of the amplifier and frequencychanger 20, in the order named, and constituting frequency changer 20 and comprising a broadband intermediate-frequency amplifier 34 and an A. V. C. rectifier 35, the latter being connected by a suitable lead 36 to the grids of one or more of the tubes in the amplifier and frequency changer 20.

Neglecting for the moment the remaining control circuits of the receiver, as well as the particular features of the circuits of the main channel which are in accordance with the present invention, the system as thus far described includes the features of a conventional superheterodyne receiver. Since the operation of such a receiver is well understood in the art, a detailed explanation thereof is unnecessary. Briefly, however, signals intercepted by the antenna 2| are selected and amplified and converted into intermediate-frequency signals in a conventional manner in the amplifier and frequency changer 20. Theintermediate-frequency signals are thereupon selected and translated by the amplifier 23, limiter 24, and selector 25, to the amplitude-detector 26, wherein the audiofrequency signal is derived. The latter signal is further'amplified and translated, by way of amplifier 21, attenuator 28, which may be adjusted to give the desired volume output, and amplifiers 29 and 30, to the loud-speaker 3| for reproduction.

Any suitable adjustable selector system, per se, may be employed in the amplifier 23. As will be described hereinafter with reference to Fig. 4, however, in the preferred embodiment of the invention, this amplifier comprises an arrangement whereby the amplification therein, as well as the shape and width of its band-pass characteristic curve, may be adjusted by varying biasing potentials applied to control electrodes of its tubes as a function of the amplitude of the received sired signal as well as the amplitudes of undesired signal disturbances. More particularly, this amplifier comprises circuits so arranged that the width of the band of frequencies passed thereby is controlled directly, and the gain is controlled inversely, in accordance with variations of a positive biasing potential applied to a control terminal thereof, while the band width. is controlled directly in accordance with a negative-biasing potential appliedto another control terminal thereof. fin order then to produce the required positive-blasing potential for automatically controlling the amplification in, and adjusting the width of the band of frequencies passed by, the amplifier 23, there is provided a broad-band in,- termediate-frequency amplifier 32 with its input circuit coupled to the output circuit of the limiter 2s and a rectifier 33 coupled to the output circuit of the amplifier 32. This amplifier and rectifier may be of conventional design to develop a unidirectional voltage which ls variable in accordance with the intensity of the desired and undesired signal voltages supplied thereto. The unidirectional voltage from the rectifier 33 is applied positively by a suitable lead 33:: to reduce an initial negative-bias potential on the first above-mew tioned control terminal of the amplifier 23, thereby to reduce the gain of, and increase the width of the band offrequencies passed by, this amplifier directly in accordance with this unidirectional voltage. The control-bias voltage developed by the rectifier 33 being proportional to the ampl tude of the total signal input thereto, a substantially uniform desired signal output is obtained from the amplifier 23.

output of the band of frequencies including not only the desired,

signal, but also .all undesiredslgnals which are passed by the radio-frequency amplifier and frequency changer 20 and which have sufilcient amplitude to be capable of overloading the frequency changer or causing interference and, preferably, it is most responsive to undesired signals on adjacent channels. The rectifier 35 is designed in a conventional manner and operates to develop a unidirectional bias voltage proportional to the amplitude of the total signalvoltage proportional to the amplitude of the total signal voltage supplied thereto. Since, as explained above, this bias voltage is effective to adjust the amplification in unit 20 inversely in accordance with the signal input to the rectifier 35, in the presence of interfering signals the amplitude of the signal input to the amplifier 23 and to the amplifier 32 is reduced, with resultant contraction of the band width of amplifier 23.

The intermediate-frequency limiter 24 comprises a circuit so designed as to cut off peaks of noise impulses, the amplitudes of which exceed the peak value of the desired signal. The peak value of the desired signal is twice the peak value of its carrier and the limiter is, therefore, designed to operate at that level. The limiter thus serves to prevent noise peaks from interfering with the operation of the control circuits which are coupled toits output circuit.

The moderately sharp intermediate-frequency selector 2! may be of conventional design and serves to provide more selectivity in the main channel than that of the control channel comprising amplifier 32. Further, with such an arrangement,the amplitude of the signal applied to the detector 26 is substantially reduced by the selector 2% when the receiver is slightly of! tune which is, of course, desirable. The attenuator 28 is a conventional manual volume level control, such as is employed in ordinary receivers having auto matic amplification control.

Referring now to the control circuits of the,

cciver is being tuned, there is connected to the output circuit of the intermediate-frequency amplifier 32, in the order named, an intermediatefrequency limiter 37, a broad-band intermediatefrequency amplifier 38, a frequency-modulation detector 39, and a full-wave rectifier All. The output circuit of the rectifier 40 is connected by a lead Illa to a control-bias terminal of the sharp cutoff audio-frequency amplifier 21. The limiter $3 is adjusted to operate at a level so low as to compress the carrier and modulation down to a constant amplitude which is substantially less than the amplitude which the carrier would have if the limiter were omitted. This provides not only a uniform carrier'amplitude for the frecufency-modulation detector 33, but also substanily removes the amplitude modulation, which undesired in frecuencmmoduiation detection. limiter, while desirable, is not, sever, essential. in the operation of part of the system, the amplified by the am- 'plifier 35 and the undesired disturbances appearing as frequency modulation on the desired signal are detected by the detector 39. The detected potential from the detector 39is rectified by the rectifier 40 in such manner as to produce a negative-biasing potential whenever the desired signal carrier is frequency-modulated to a predetermined degree as when the receiver is detuned by a substantial amount on either side of resonance, and this biasing potential is so applied to the amplifier 21 as to quiet the the reproduction of the desired signal as well as undesired signal disturbances.

The output circuit of the frequency-modulation detector 39 is also coupled to two additional arrangements for controlling other characteristics of the receiver in response to other undesired signal disturbances One arrangement comprises, connected in the order named an audiofrequency impulse selector 4|, anaudio-frequency amplifier 42,and a full-wave rectifier 43 having a negligible time constant. The selector 4| is designed to be somewhat more responsive to higher audio frequencies than to lower frequencies, since noise, such as rapidly recurring impulses or steady background noise, ordinarily has most of its power in the higher audio-frequency range. The selector 4|, therefore, is preferably of the high-pass type and, further, has a so-called negative delay". The last-mentioned characteristic accelerates its response to sharp impulses and thereby renders the control for such impulses more effective.

After selection in the selector 4|, the undesired signal disturbances are further amplified in the amplifier 42 and instantaneously rectified with negligible time lag by the rectifier 43, thereby to develop transient negative-biasing potentials which closely follow the undesired signal disturbances.- This negative transient potential is applied directly, by way of a suitable lead 44, to the audio-frequency amplifier 30 instantaneously to quiet the receiver during a sharp impulse. The negative-biasing potentials developed by the rectifier 43 are also applied to the negative control terminal of the intermediate-frequency amplifier 23 and to two biasing terminals of the audio-frequency amplifier 29 by way of leads 45,. 45, and 41, which include resistors 45a, 45a, and 4111, respectively, for smoothing out these potentials and provide steady biasing potentials, as distinguished from the transient potential applied over the lead 44. The left-hand biasing terminal of the amplifier 29 represents a connection whereby the gain and the band width may be decreased by the application of a negativebias voltage. The right-hand terminal represents a connection whereby the band width may be decreased and the gain increased by the application of a negative-bias voltage. The biasing potentials applied to these terminals are developed by control circuits of the present invention, as hereinafter further described, and are proportioned to cause negligible resultant change in gain while adjusting the band width. There fore, the steady potentials derived from rectifier 43 serve to reduce the. width of the band of frequencies passed by the amplifiers 23 and 29 when the average noise is sufilciently great to make such control desirable.

The finalcontrol arrangement, which is also coupled to the output circuit of the frequency detector 39, includes, in the order named, a beat-note selector 49, a beat-note amplifier 45, and a beat-note rectifier N. The selector 48 is selected beat notes are receiver or suppress designed to be most responsive at frequencies of approximately 10 kilocycles and higher and thus serves to select beat notes produced by carriers on adjacent channels 10 kilocycles or more at either side of the desired signal carrier. ThQe thereupon amplified by the amplifier 49 and rectified by the rectifier 50 to develop steady biasing potentials. These potentials are applied, by way of leads 5|, 52, and 53, to supplement the biasing potentials applied from the rectifier 43 by way .of the leads 45, 46, and 41, respectively, and to ensure sufficient contraction of the frequency bands passed by the amplifiers 23 and 29 substantially to avoid the efifects of beat-note interference. Diodes 51a, 52a, and 53a are preferably included in the leads 5|, 52, and 53, respectively, so that the connections will not by-pass the biasing voltages from the rectifier 43 when the latter are the greater. While the diodes may be replaced by isolating resistors, the use of the diodes renders itpossible to disconnect the arrangement including the beat-note selector amplifier and rectifier without affecting the normal operation of the. remainder of the receiver.

In Fig. 4 there are illustrated in detail representative circuits embodying the present invention which may be employed in the receiver exemplified in Fig. 3, similar reference numerals being applied to corresponding stages in the two figures. It will, of course, be understood that these detailed circuits are shown simply by way of illustrating one satisfactory embodiment of the invention and that any other suitable equivalent circuits may be substituted therefor, if desired. As illustrated, the amplifier 23 comprises a tuned input circuit 55 loosely inductively coupled to the output circuit 56 of the unit 20 and a. tuned output circuit 51 loosely inductively coupled to a tuned input circuit 58 of the intermediate-frequency limiter 24. The input and output circuits 55 and 51 are coupled both forwardly and backwardly by means of directive coupling means, such as vacuum tubes 59 and 60, respectively, input electrodes of the tube 59 being coupled to the circuit 55 and its output electrodes coupled to the circuit 51 and the input electrodes of the,tube 50 being coupled to circuit 51 by means of a winding 62, and its output electrodes. being inductively coupled back to the input circuit 55 by means of a winding 63. Each of, the circuits 55, 51, and 58 is tuned to the intermediate-frequency at which the receiver is operated. Preferably, the circuits 55 and 51 are relatively sharply tuned and have a very low power factor, while the circuit 55 is broadly tuned and has a higher power factor.

By utilizing the tube 59 in the forward coupling means of the amplifier 23 with no phase reversals in its couplings with the circuits 55 and 57, the alternating voltages appearing across, the circuit 51 are substantially reversed in phase with respect to the voltages across the input cir cuit 55 at frequencies in the vicinity of the resonant frequency of the circuits, at which frequencies these circuits are of high impedance and are substantially resistive. A second phase reversal is secured in the backward path in the tube 80 and a third reversal is secured in the coupling between the inductance 63 and the input circuit 525, so that the voltages impressed on the input circuit 55 through this'backward coupling path are substantially reversed in phase, at the frequencies indicated, with respect to the input voltage directiy impressed on this circuit.

and the system is degenerative to a maximum degree at the intermediate frequency.

At frequencies above the resonant frequency of the circuits 55 and 51, these circuits are capacitively reactive so that the voltages at these frequencies across the circuit 51 lag the input voltages by phase angles approaching 90 degrees. The feed-back voltages at these frequencies impressed on the circuit 55 are similarly retarded by an additional phase angle also approaching 90 degrees, so that the feed-back voltages approach phase coincidence with the input voltages at these frequencies and the coupling system is regenerative therefar. At frequencies below the resonant frequency of the circuits 55 and 51, these circuits are inductively reactive and a similar phase shift occurs, but in opposite sense, so that the system is also regenerative at these frequencies. However, at these limiting frequencies, the impedances of the circuits 55 and 51 are much less than at resonance, the trans mission eiilciency of the amplifier 23 being reduced and the amplitude of the feed-back voltage also being reduced, so that, while the system is regenerative, it is entirely stable in operation. At frequencies intermediate the limiting frequency just described, the feed-back voltages have intermediate phase angles with respectto the input voltages and the feed-back characteristic of the system has a gradual transition from degeneration to regeneration. Hence, the resultant reduction in amplitude of the frequencies near the intermediate frequency and the increase in amplitude of the frequencies substantially above and below the intermediate frequency impart to the system a band-pass frequency characteristic like that of inductively coupled tuned circuits.

By adjusting the forward and backward coupling reactions between the resonant input and output circuits of the amplifier 23, the amplification obtained between the terminal circuits thereof and the shape of the width of the bandpass characteristic may be controlled as desired. Such coupling control is procured, in accordance with the present invention, by varying the bias potentials applied to the control electrodes of the tubes 59 and 60, a positive biasing potential being applied to the tube 60. from the rectifier 33 and a negative-biasing potential being applied to. the tube 59 from the rectifiers 43 and 50, to control the amplification and band-pass characteristic of the amplifier 23, as described above.

The intermediate-frequency limiter 24 com-- prises the tuned input circuit 58 and a diode limiter tube 64 connected thereacross. A source of biasing voltage, indicated by a battery 54a, is included in the cathode circuit of the diode 64 and provides this tube with a positive cathode bias, the peak value of which is equal to the peak voltage of the signal or twice the peak voltage of the unmodulated carrier itself, was to by-pass the peaks of noise impulses which exceed this value and prevent their interfering with the operation of succeeding stages of the system. An

The stages 25, 26, 21, and 28 are substantially I conventional in design and operation. The selector 25 comprises a moderately sharp parallel resonant circuit 66 having its inductance winding coupled to the winding of the limiter 24. The

be controlled in the usual manner.

The adjustable band-pass audio-frequency amplifier 29 comprises an arrangement the construction and operation of which are somewhat similar to that of the amplifier 23, but which is adapted for audio-frequency operation instead of intermediate-frequency operation. The amplifier 29 includes a forward coupling tube H and a backward coupling or feed-back tube 12. Attenuation of the frequencies is provided by means of a resistance-reactance network which limits the transmission of very low or very high audio frequencies and in this respect the arrangement is quantitatively quite different from that of the amplifier 23. More particularly, the input circuit of the tube ll includes a series resistor Na and series and parallel condensers l3 and H, respectively. These condensers attenuate the low and high audio frequencies, respectively. The anode circuit tube 1| includes a primary winding 15 of an audio-frequency transformer it and a trap circuit, comprising a winding 'i'l and condenser 18 in series, connected across the winding I5. The trap circuit is tuned to 10 kilocycles, which is the usual frequency difference between carriers on adjacent channels. A part of the voltage developed across the winding i5 is coupled by way of a blocking condenser 19 to the grid of the tube 12. A resistor 8| is included in the cathode circuit of the tube I2 so that an audio-frequency voltage is built up thereaoross. This voltage is coupled back by way of a resistor 82 to the grid of the tube Ii to secure feedback which, due to phase shifts effected by the forward and backward couplings, similar to those referred to in connection with the amplifier 23, is degenerative at the mean audio frequencies, but regenerative at lower and moderately higher audio frequencies. The feedback is degenerative also at frequencies higher than the resonant frequency of the trap circuit and this is a desirable condition, since it is desired to attenuate such high frequencies. A grid-leak resistor 83 is connected to the control grid of the tube H and this resistor and the resistor 80 are by-passeci to ground by way of audio-frequency by pass condensers 84 for the purpose of smoothing out biasing potentials applied to the control grids of the tubes H and 12, as hereinafter explained. These tubes are preferably similar and of the gradual cutoif type. The cathode of each of the tubes II and I2 isinitially biased positively to prevent the.

flow of grid current on the signal peaks,

The audio-frequency amplifier 29, under the conditions just described, is adjusted to provide the maximum desired band width by virtue of the feedback through the tube 12. The band width is reduced by the application of negativebiasing potential to the grids'of both of the tubes, less potential being applied to the tube H than to the tube 12, and these potentials being proportioned to maintain the overall gain substantially constant at frequencies in the middle of the audio-frequency range.

The amplifier 30 comprises a conventional push-pull arrangement including sharp cutoilf vacuum-tube amplifiers 85 and 86 having their control electrodes connected to the secondary winding 81 of the audio-frequency transformer 16. The output circuits of tube 86 are coupled in theusual manner by way of an audio-frequency transformer ca m the loud-speaker of the system.

The intermediate-frequency amplifier 32 and rectifier 33 are substantially conventional circuit arrangements and preferablyinclude a pentode amplifier 39 having its control grid coupled to the limiter circuit 24 by way 01 a suitable coupling condenser 90 and leak resistor SI and including .an inductance winding 93 in its anode circuit. The rectifier 33 comprises a diode 94 having an input circuit including a winding 95 coupled to the winding 93 and a load circuit ineluding a resistor 96 and by-pass condenser 91. The amplifier 32 is broadly selective by virtue of its coupling to the limiter 24. A unidirectional voltage is developed by the rectifier 33 across the resistor 96 which is variable in accordance with the intensity of desired and undesired signal voltages in the output of the limiter circuit. A portion of the unidirectional voltage across resistor 96 is applied by way of the resistor SI as a positive grid bias to the control grid of the amplifier 89 to provide a reverse automatic amplification control, that is, to increase its amplification with increased bias voltage and thereby to secure a substantially level amplification control. In United States Letters Patent No. 2,050,679, granted August 11, 1936 on an application of Harold A. Wheeler, there is described in detail a system embodying a reverse automatic amplification control arrangement similar to that illustrated, and reference may be had to that patent for a detailed disclosure of the principles involved in such a control.

By means of a second connection to the resistor 96 .of the rectifier 33 and by way of a suitable lead 33a including a filter resistor 99 and the feed-back winding 62 of the amplifier 23, a positive-bias voltage is also applied to the control grid of the backward coupling tube of the amplifier 23 to increase the degenerative feedback and thus to reduce the gain and increase the band width 01' this amplifier with increasing signal input to the rectifier 33, as described above. The net result of this arrangement is that there is obtained a substantially uniform carrier input to the limiter 24 for obtaining satisfactory limiting action and for translation to the subsequent stages of the system.

Referring now to the control arrangement including the stages 31, 38, 33, and 40, the limiter 31 comprises a broadly tuned resonant circuit I00 of high impedance loosely coupled by means oi a small condenser IM to the output circuit of the amplifier 32. Connected across the circuit I00 is a diode I02 having a battery I03 included in its cathode circuit to provide a small positive bias for determining the signal level of the limiting operation. Due to this bias, the voltage across the circuit I00 is limited to a value which is considerably less than the carrier voltage which would otherwise exist across the circuit. "The amplifier 38 comprises a pentode amplifier'tube I I04 having its input circuit connected across the limiter circuit I00, I02. The frequency-modulation detector 38 comprises a frequency-discrimirating network I06 and diode rectiflers I 01 and I08. The discriminating network includes three inductance windings I03, I I0, and II I, of which end to an intermediate tap on the winding I II and its opposite end is connected to the cathodes of diodes I01 and I08 by way of load circuits including resistors Ill and intermediate-frequency by-pass condensers H5, as shown, the

junctions between the condensers H5 and resistors I I4 being by-passed to ground by condenser I I8. Each of the two rectifiers thus receives the voltage 01' the winding III and half the voltage of the winding III, the winding I II being connected differentially in the two rectifier circuits providing the well-known type 0! frequency discriminator dependent upon the phase relations of the primary and secondary voltages to obtain asymmetrical frequency characteristics.

A center tapped resistor I I1 is connected across the load circuits of the two rectifiers and its center tap is grounded so that equal and opposite rectified voltages relative to ground are obtained from its terminals. While this expedient is ordinarily not employed in frequency detectors, it is utilized here in order to obtain an output voltage which is balanced relative to ground.

The full-wave rectifier 40 comprises two diodes II 3 and I20. having their cathodes connected to opposite ends of resistor II! by way of series resistor I2I, by-passed to ground for audio frequencies by condensers I22. A battery I2! is included in the rectifier circuit, as shown, in order to suspend the operation of diodes Ill and I20 until the rectified output exceeds a. predetermined small threshold value. The full-wave rectified output of diodes II! and I20 appears across a resistor I20 in their common load circuit.

When the receiver is properly tuned, the output of the detector 39 comprises only audio-frequency components which do not reach the full-wave rectifier 40. As the receiver is substantially detuned, however. undesired signal disturbances are developed as the result of the dissimilarity oi the sidebands passed by the system, as well as the variation in the locally generated oscillation frequencies, and these disturbances appear as both amplitude and frequency modulation on the desired signal carrier. The unidirectional potential in the output of the detector 3!, therefore, increases beyond the suspender bias and a unidirectional voltage is developed by the rectifier 40 across the resistor I20. This potential is applied by way of a suitable lead 00a including a series filter resistor I20 to the control grid of the tube 00 of the sharp cut-oil audio-frequency amplifier 21, thereby serving to reduce the conductance of this tube to zero and thus quiet the receiver when the receiver is substantially on tune.

In the control arrangement including the stages ll, 42, and 43, the audiodrequency impulse lelector 4| comprises the primary winding I 20 of a transformer I30 having a center tap connected to ground, shunt resistors I 3! connected between quency range. The network is somewhat more responsive to noise'than to signal modulation which might accidentally get into these control circuits.

The audio-frequency amplifier 42 comprises a vacuum tube I33 having its input circuit coupled to the secondary winding I34 of transformer I30,

and the primary winding I35 of an audio-frequency transformer I36 included in its output circuit. The full-wave rectifier 43 comprises two diodes I39 and I39 connected to the secondary winding I40 of the transformer I36 and having a load circuit comprising resistors HI and I42. A delay bias is applied to the cathodes of the diodes I38 and I39 from a suitable source, such as a battery I43. Undesired signal disturbances selected by the selector I and amplified .by the amplifier 42 are rectified in the rectifier 43 with negligible time delay to develop unidirectional voltages across the resistors HI and I42. The

vvoltage developed across the two resistors is applied by way of the lead 44 directly to the control grids of the amplifier tubes 86 and by way of the leads 45 and 46 including resistors 45a and 46a, respectively, to the control grid of the forward coupling tube 59 of the amplifier 23 and to the control grid of the feed-back coupling tube 12 of the audio-frequency amplifier 29. The potential developed across the resistor I42 is applied by way of the lead 41 including the resistor 41a to the control grid of the forward coupling tube II of the amplifier 29. In this manner the biasing voltages developed by the rectifier 43 in response to sharp noise impulses are applied with negligible time delay to the amplifier 30 to reduce its conductance substantially to zero and thereby quiet the receiver for the duration of the impulses, while the biasing voltages developed in response to the average noise received controls the amplifiers 23 and 29 to reduce the width of the frequency bands passed thereby sufliciently to substantially avoid the effect of such noise. as described above.

In the arrangement including the stages 48, 49,

and 50, the beat-note selector 48 comprises a transformer I44, the primary winding I45 of which has a center tap connected to ground and is connected across the output resistor II! of the detector 39 by way of coupling condensers I46. The secondary winding I41 of transformer I44 is connected in series with an uncoupled inductor I48 and a condenser I50 to form. a circuit most responsive at frequencies of about 10 kilocycles and higher for selecting beat notes produced by carriers in adjacent channels 10 kilocycles or more at either side of the carrier, as previously explained. More particularly, the beat-note selector 49 has the characteristic of a high-pass filter which is unresponsive at frequencies substantially below 10 kilocycles, has a peak at 10 kilocycles, and is also responsive at higher frequencies, such as 20 or 30 lrilocycles.

The amplifier 49 comprises a conventional amplifier tube II, its input circuit being connected across the beat-note selector circuit and its anode circuit including-the primary winding I52 of a transformer I49. The rectifier 50 comprises a conventional diode L54 connected to the secondary winding I55 of transformer I49 and having a load circuit including resistors I56 and I51 shunted by a condenser I58.

Beat notes resulting from the beating of adjacent undesired signal carriers with the desired signal carrier and frequencydetected by the detector 39 are selected by the selector 48, amplified impulse selector 4I-,

by the amplifier 49, and rectified by the rectifier 50 to develop negative unidirectional biasing potentials across the resistors I56 and l51. The potentials developed across these two resistors is applied by way' of the leads 5I and 45 to the control grid of the forward coupling tube 59 and ,by way of the leads 52 and 46 to the control grid of the backward coupling tube 12 in the amplifier 29. The potential developed across the resistor I56 is applied by way of leads 53 and 41 to the control grid of the tube 1I. These biasing potentials serve to supplement the biasing potentials applied from the rectifier 43 to ensure suflicient contraction of the widths of the frequency band passed by the amplifiers 23 and 29' responsive. The diodes 5Ia, 52a, and 53a are' included in the biasing leads and function as.

described with reference to Fig. 3.

All of the detectors employed in the system are preferably linear detectors with respect to their essential characteristics. In the amplitude detectors, linear rectification is obtained in the usual manner by loading the diodes with high resistance. The frequency-modulation detector 39 should have, in addition, substantially linear discriminator characteristics over a frequency range of about 20 kilocycles centered on the intermediate frequency. The output of the detector 39 is then proportional to the frequency modulation as long as it does not exceed kilocycles from the mean.

Operating potentials are supplied for the various tubes of the system from suitable sources in conventional manner, batteries having been indicated in the drawings for this purpose.

Since the particular operations of the various control arrangements have already been described in connection with the above detailed description of the apparatus, repetition thereof is unnecessary. To summarize, however, when undesired signal disturbances of the various different kinds are received, they frequency-modulate the desired signal and this signal is amplified by the amplifier 32, limited as to its amplitude by the limiter 31. further amplified in the amplifier 38, and the frequency 'modulation is detected by the detector 39. When these undesired signal disturbances are such as are caused by improper tuning of the receiver, they are rectified by the rectifier 40 and applied to the amplifier 21 to cut off the conductance thereof and substantially suppress the reproduction of both the undesired signal components and the desired signal during such mistuning. When the signal disturbances are electrical transients or are of the continuous type and distributed throughout the modulationfrequency range, they are translated from the frequency detector 39 to the audio-frequency are selected therein and thereupon amplified in the amplifier 42, and rectified in the rectifier 43. Control voltages are thus developed which are proportional to the undesired disturbances of the continuous type and these voltages are applied to the amplifiers 23 and 21 to contract the width of the bands of frequencies passed by these stages of the system in accordance with the amplitude of such undesired signal disturbances, thereby to reduce the efiects thereof. Control voltages are also thus developed by the rectifier 43 responsive to the electrical transients of short duration and this voltage is applied directlyto the amplifier 30 to cut off this amplifier and thereby suppress the reproduction of both the desired and undesired signals for the duration of such transients. 1 1

When the undesired signal disturbances are such as are caused by the presence of an undesired signal on a carrier frequency adjacent the desired signal-carrier frequency, or, in other words, such as result in 10 kilocycle beat notes, these beat notes are detected by the frequency detector 39, selected by the selector 48, amplified by the amplifier 49, and rectified by the rectifier 50. The unidirectional voltages thus developed are applied to supplement the 'control voltages developed by the rectifier 43 and applied to the amplifiers 23 and 29 and effect the required contraction of the width of the band of fre quencies passed by these amplifiers to avoid the effects of such beat-note interference.

While there have been described what are at present consideredto be the preferred embodiments of this invention, it will be obvious to those skilledin the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is. claimed is:

A modulated-carrier signal receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and of a second characteristic type by undesired signal disturbances, means for selectively' detecting and utilizing said selectedcarrier modulation of the first type to reproduce the desired signal, and means for selectively detecting and utilizing only said selected-carrier modulation of the second type so to control the characteristics of the receiver as to reduce the eifects' of said undesired signal disturbances.

A modulated-carrier signal receiver comprising means for selecting a carrier having am-' plitude modulation-representing a desired signal and subject to both amplitude modulation and frequency modulation by undesired signal disturbances, means for selectively detecting and utilizing said amplitude modulation to reproduce the desired signal, and means -for selectively detecting and utilizing only said frequency modulation so to control the characteristics of the receiver as to reduce the efi'ects of said undesired signal disturbances.

3. A modulated-carrier signal receiver comprising means i'or selecting a carrier having i'requency' modulation representing a desired signal and subject to both irequency and amplitude modulation by undesired signal. disturbances.

means for selectively detecting and utilizing said frequency modulation to reproduce said desired signal, and means for selectively detecting and utilizing only said amplitude modulation so to control the characteristics of said receiver asto reduce the eflects of said undesired signal disturbances.

A modulated-carrier signal receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and or a second characteristic type by substantially continuous undesired signal disturbances, means including a band-pass selector for selectively detecting, and utilizing said modulation of the first type to reproduce thede sired signal, and means lelectively responsive 7 lectively detecting and utilizing said modulation of the first type to reproduce the desired signal, and means selectively responsive only to modulation of said carrier of said second type for ad-.-

justing the width of the band of frequencies passed by said selector substantially to reduce the eifect of said undesired signals.

A modulated-carrier signal receiver comprising means for selecting a carrier having amplitude modulation representing a desired signal and subject to both amplitude and frequency .modulation by substantially continuous signal disturbances, means including a band-pass selector for selecting, detecting, and utilizing said amplitude modulation to reproduce the desired signal, and means selectively responsive only to said frequency modulation of said carrier for adjusting the width of the band of frequencies passed by said selector in accordance with said undesired signal disturbances, thereby to reduce the effects thereof.

7. A modulated-carrier signal receiver comprising a main signal-translating channel including means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to'modulation of said first type and of a second characteristic type by undesired signal disturbances, means for selectively detecting and utilizing said selected-carrier modulation of the. first type to reproduce the desired signal, adjustable means for reducing the efiects of said undesired signal disturbances, an

auxiliary signal-translating channel comprising means for selectively detecting only modulation of said carrier of said second type, and means for utilizing. said selected-carrier modulation of said second type for controlling said adjustable means.

A modulated-carrier signal receiver comprising a main signal-translating channel including means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation oi. said first type and of a second characteristic type by substantially continuous undesired signal disturbances, an adjustable band-pass selector for translating said desired signal and means for detecting and utilizing said modulation of said first type to reproduce the desired signals, an" auxiliary signal-translating channel coupled to said selecting means and including means for selectively detecting only modulation 01' said carrier of said second type, and means for utilizing said modulation of said second type for so adjusting the width of the band of frequencies passed by said selector as to reduce substantially the eflects of said disturbances.

9. A modulated-carrier signal receiver compris ing a main signal-translating channel including means for selecting a carrier having amplitude modulation representing a desired signal and subject to both amplitude modulation and frequency modulation by substantially continuous undesired signal disturbances; an adjustable band-pass selector for translating said desired signal, means for selectively detecting and utilizing said amplitude modulation to reproduce the desired signal. an auxiliary signal-translating channel coupled to said selecting means and including means for selectively detecting only said frequency modulation, means for rectifying the detected frequency modulation to develop a control-bias voltage variable in accordance with the amplitude of said undesired signal disturbances, and means for utilizing said bias voltage for adjusting the width of the band of frequencies passed by said selector substantially to reduce the efi'ects of said disturbances.

10. A modulated-carrier signal receiver comprising a main signal-translating channel including means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and of a second characteristic type by substantially continuous undesired signals on the carrier adjacent the desired signal carrier, an adiustable band-pass selector for translating said desired signal, means for detecting and utilizing said modulation of the first type to reproduce the desired signal, an auxiliary signal-translating channel coupled to said selecting means and including means responsive only to modulation of the carrier of said second type for developing a control-bias voltage variable iii accordance with said undesired carrier, and means for utilizing said bias voltage for adjusting the width of the band of frequencies passed by said selector substantially to reduce the effects thereof.

11. A modulated-carrier signal receiver comprising a main signal-translating channel including means for selecting a carrier having amplitude modulation representing a desired signal and subject to both amplitude and frequency modulation by substantially continuous undesired signals on carriers adjacent the desired signal carrier, an adjustable band-pass selector for translating said desired signal, means for detecting and utilizing said amplitude modulation to reproduce the desired signal, an auxiliary signaltranslating channel including means for selectively detecting only said frequency modulation to develop beat-note signals variable in accordance with the amplitude of said undesired signals, and means for utilizing said beat-note signals to adjust the width of the band of frequencies passed by said selector substantially to reduce the effect of said undesired signals. a

12. A modulated-carrier signal receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subje t to modulation of said first type and of a second characteristic type by undesired signal disturbances of a discontinuous character, means for selectively detecting and utilizing said modulation of said first type to reproduce the desired signal, and means responsive only to said modulation of said carrier of said second type for substantially suppressing the reproduction of both the desired signal and said undesired signal disturbances for the duration of said disturbances.

13. A modulated-carrier signal receiver comprising a main signal-translating channel including means for selecting a carrier having a first characteristic type modulation representing a desired signal and subject to modulation of said first type and of a second characteristic type by undesired signal disturbances of a discontinuous character and means including an amplifier for detecting, amplifying, and reproducing the desired signal, an auxiliary signal-translating channel coupled to said selecting means and including means for selectively detecting only modulation of said carrier of said second type, and means having negligible time delay for utilizing said modulation of said second type to reduce the conductance of said amplifier substantially to zero, thereby substantially to suppress the reproduction of both the undesired disturbances and said desired signal for the duration of said disturbances.

14. A modulated-carrier signal receiver comprising tunable means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and of a second characteristic type by undesired signal disturbances caused by mistuning of said selecting means, means for selectively detecting and utilizing said modulation of the first type to reproduce the desired signal, and means selectively responsive to modulation of the second type for substantially suppressing tl'l reproduction of both the undesired signal disturbances and said desired signal when said selecting means is mistuned.

15. A modulated-carrier signal receiver comprising a main channel including tunable means for selecting a carrier having amplitude modulation representing a desired signal and subject to both amplitude and frequency modulation by discontinuous undesired signal disturbances caused by mistuning of said selecting means, means including an amplifier for detecting, amplifying, and utilizing said amplitude modulation to reproduce the desired signal, a control channel coupled to said selecting means and including means for selectively detecting only said frequency modulation, and means having negligible time delay for utilizing said frequency modulation to reduce the conductance of said amplifier substantially to zero, thereby substantially to suppress the reproduction of both the undesired signal disturbancesand said desired signal when said selecting means is mistuned.

16. A modulated-carrier signal receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and of a second characteristic type by electrical transients, means for selectively detecting and utilizing said modulation oi. said first type to reproduce the desired signal thereby incidentally reproducing said transients, and means selectively responsive only to modulation of said carrier of said second type substantially to suppress the reproduction of both the transients and said desired signal for the duration of said transients.

17. A modulated-carrier signal receiver comprising a main channel including means for selecting a carrier having amplitude modulation representing a desired signal and subject to both amplitude and frequency modulation by electrical.

transients, means including an amplifier for ampliiying, detecting, and utilizing said amplitude modulation to reproduce the desired signal thereby incidentally reproducing said transients, a control channel coupled to said selecting means and including means for selectively detecting only said frequency modulation, and means having negligible time delay for utilizing said frequency modulation to reduce the conductance of said amplifier substantially to zero, thereby substantially to suppress the reproduction of both said transients and said desired signal for the duration of said transients.

18. A modulated-carrier signal receiver comprising means for selecting a carrier having a first characteristic type of modulation representing a desired signal and subject to modulation of said first type and ofa second type by substantially continuous undesired signal disturbances and by undesired signal disturbances of a discontinuous character, means including a band-pass selector and an amplifier for selecting, detecting, amplitying, and utilizing'said modulation of said first type to reproduce the desiredsignal, means selectively responsive only to modulation of said carrier of the second type by continuous undesired signals for adjusting the width of the band of frequencies passed by said selector substantially to reduce the effects of said continuous disturbances, and means 'responsive only to modulation of said carrier of said second type by said disturbances of a discontinuous character for controlling said amplifier substantially to suppress the reproduction of all desired and undesired signals for the duration of discontinuous signal disturbances.

HAROLD A. WHEELER. 

